1. Field of Invention
The invention relates generally to lasers and more particularly, to techniques for controlling amplifier power in master oscillator power amplifier (MOPA) fiber lasers.
2. Description of Related Art
Pulsed lasers are lasers which emit light not in a continuous mode, but rather in the form of optical pulses. Depending on the pulse duration, pulse energy, pulse repetition rate and wavelength required, different methods for pulse generation and types of pulse lasers may be used. For example, various types of Q-switched lasers can be used to generate nanosecond pulse durations. High pulse energies are achievable with solid-state bulk lasers. For smaller pulse energies, a microchip laser or a fiber laser can be suitable. For boosting the average power (particularly of high repetition rate pulse trains with moderate pulse energies), high-power fiber amplifiers can be used.
Power scaling of a laser is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design. Usually, power scaling requires a more powerful pump source, stronger cooling, and an increase in size. It may also require reduction of the background loss in the laser resonator and, in particular, in the gain medium. The most popular way of achieving power scalability utilizes a “MOPA” (Master Oscillator Power Amplifier) approach. The master oscillator produces the desired pulse train, and an optical amplifier is used to increase the power of the beam while preserving its main properties. The master oscillator has no need to be powerful and has no need to operate at high efficiency because the efficiency is determined mainly by the power amplifier.
Pulsed MOPA fiber lasers require control of the population inversion, that is, the stored energy, in order to perform at low frequency and when gated on and off. The amount of population inversion buildup between pulses directly determines the energy content of the output pulse of the laser. The buildup reaches equilibrium for a given medium when stimulated and spontaneous deexcitation balances off the pump excitation. When the laser is operated at a high pulse repetition frequency, the laser can safely reach this equilibrium with continuous pumping. In this case, the pulse power can be controlled by regulating the level of the pump power. For MOPA lasers to operate at low pulse repetition frequency, pump control is required as the equilibrium would be at stimulated and spontaneous deexcitation levels which would either cause spontaneous lasing or peak power levels causing catastrophic optical damages to the amplifier.
Prior techniques utilizing pump control often do so through a lookup table, from which an estimate of the correct amount of pumping energy and duration to apply per fired pulse can be found in order to control the pump. The lookup table is generated by calibrating the laser across its operating range of pulse repetition frequencies, pulse widths and power settings. The pump power and duration for the lookup table depend on many variables, including the pulse profile, power set-point, pulse repetition frequency, and on/off gating history. Electronics are used to implement the lookup table and apply the correct pump power and duration for the particular operating points. However, the lookup table is a poor solution, since the process of creating it is time consuming, laborious, and difficult. Also, the laser is still limited to the operating points contained in the lookup table. Furthermore, the lookup needs to happen in real time, placing demanding requirements on the electronics and inevitably forcing a compromise between timing and accuracy.
Given the limitations of calibration approaches, most current systems avoid the problem of controlling the inversion by limiting operation to high frequencies, as high frequencies do not require pump control. However, limiting the operating frequency range to only high frequency limits the number of applications in which the laser can be used. For example, the maximum measurable distance in a light detection and ranging (LIDAR) system can be limited by pulse ambiguity if low frequency pulses are not used.
Feedback techniques have also been implemented to control pump power. FIG. 1 illustrates a conventional pump control system 100 utilizing a feedback technique. Here, the pump control system 100 comprises a laser system 110, a controller 120, and an optical sensor 130. The laser system 110 comprises, among other things, a controllable pump energy source (e.g., laser diode) coupled to a gain medium. The laser diode generates radiation which causes a population inversion within the gain medium to drive the laser. The sensor 130 (e.g., photodetector) is coupled to the gain medium to detect the magnitude of fluorescence from the gain medium, which is related to the amount of population inversion in the gain medium. The sensor 130 generates a signal reflecting the magnitude of the detected fluorescence. The sensor signal is fed to the controller 120 in order to approximate the error between the actual laser pulses emitted from the laser system 110 to those desired in a particular application. Based on the approximated error, the controller 120 adjusts the pump energy source accordingly to better achieve the desired output laser pulses. For example, the controller 120 may adjust the magnitude and switching rate of a voltage applied to a laser diode from a set point voltage value. U.S. Pat. No. 5,018,152 to Linne, the entire disclosure of which is incorporated by reference herein, discloses such a feedback technique in a Q-switched laser system.
A drawback of utilizing such a feedback technique is that it relies on optical measurement of the magnitude of fluorescence from the gain medium. The fluorescence measurement gives a value from which the inversion has to be inferred. Linne suggests measurement of the transverse fluorescence, which for short gain medium, e.g., a rod, can give a reasonable estimation of the inversion. For an optical fiber gain medium, the transverse fluorescence value will depend on the point of measurement. The fluorescence can be measured in the longitudinal fiber direction, as core guided or as cladding guided radiation. However, in this case, the fluorescence detection device needs to tolerate the high pump or signal power also present at those locations. Even with optical filters, a system with sufficient power handling will generally lack the resolution necessary to accurately estimate the error in laser inversion. This is due to the limitations in dynamic range of converting intensity in the optical domain to voltage in the electrical domain. Moreover, the magnitude of fluorescence at some points is not an accurate measurement of the effective population inversion in the gain medium. These drawbacks with prior known approaches lead to a cumbersome system and poor control of the of the gain medium and hence output energy.
Accordingly, a need has arisen to better measure and/or estimate the population inversion found in a gain medium in order to control the power supplied to the gain medium.